A turbocompressor cascade comprises a plurality of turbocompressors, i.e. compressor turbines, between which a heat exchanger is provided for the intermediate cooling of the compressed medium, generally a gas, so that the compressed gas passing to the next compressor stage is after cooled. Generally a corresponding heat exchanger is provided following the final stage as well.
The parameters of such a turbocompressor cascade can include the intake side pressure (p.sub.1) the discharge side pressure (p.sub.2), the intake side temperature or suction temperature T.sub.s, the intermediate temperatures (or aftercooling temperatures) following cooling T.sub.ri, where i represents the stage, i.e. is 1, 2, 3 . . . , n, also referred to as backcooling temperatures, and the volume rate of flow (F) through the system.
The measured values (p.sub.l, p.sub.2, F, T.sub.s, T.sub.ri) are fed to a control device with a data storage capability, can be compared with values of a compressor characteristic operating graph (i.e. the empirically derived optimum operating characteristics in a form enabling such storage and comparison) so that when the measured values of p.sub.1, p.sub.2 and F, for example, approach limits of the compressor operating graph, i.e. the so-called pumping limits, a warning signal or controlled signal is provided to prevent continued operation at or beyond these limits or, at least, to alert operating personnel that the limits have been approached.
The reference to a compressor operating graph is intended to include a collection of stored data in any form enabling the comparison of the measured values with corresponding empirically determined conditions of intended operation capable of indicating the approach to the pumping limit. For example, the "graph" can simply be tabulated data or operating tables derived from such data or other parameters automatically calculated by computer from stored data. All of these techniques are known in the art.
The pumping limit is defined as an aerodynamic stability boundary for the operating graph which limits the utility of the turbocompressor. When operation is effected below the pumping limit, refluxing or backflow can occur in the turbocompressor which results in pressure fluctuations, temperature increases and significant increase in the pumping noise.
Operating below these limits or at the pumping limit can lead in very short order to bearing failure and damage to the turbine blades.
To avoid operation of a turbocompressor in an unstable range, i.e. at or below the pumping limit, monitoring and controlled devices are provided which, upon critical approach to or passage of the pumping limit, open blow-off valves to the atmosphere or open a bypass valve in a circulating line which connects the pressure and suction sides of the turbocompressor. In this manner, a minimum flow through the turbocompressor is maintained and operation below the pumping limit is precluded.
Intermediate-cooled turbocompressors should allow control over a wide volume range. Control elements for this purpose can include inlet or outlet guide devices or drives with variable speeds for the turbocompressors. Combinations of these controlled elements can also be provided. However, the full utilization of the entire operating graph of the turbocompressor cascade requires that the pumping limit be determinable with precision.
Conventional processes for monitoring the pump limit of multistage intermediate cooled turbocompressors ignore the influence of changes in the cooling temperature on the pumping limit. The intermediate cooling temperature, constituting the temperature of the gas at the input to each successive stage has been found to play a major role upon the location of the pumping limit in the operating field of the turbocompressor.
In operation of the turbocompressor, variations of the stage input temperatures cannot be avoided and these variations can be a function of soiling of or poorly operating coolers, variations in the flow of the cooling agents through the heat exchangers or the like.
There are also seasonal changes in the intake temperatures of the gases to be compressed.
Ignoring the temperature effect on the location of the pumping limit requires that the turbocompressor be operated within much narrower operating limits with respect to volume variations, since the approach to the actual pumping limit can never be guaranteed entirely. Thus conventional systems of this type operate with limited versatility and with loss of range.